1. Field of the Invention
The present invention relates to electron tube microwave sources, and more particularly, to a method and apparatus for extracting microwave power from a modulated, off-axis electron beam.
2. Description of Related Art
Microwave vacuum tube amplifiers generally use either velocity or density modulation of an electron beam in order to establish an AC current that is subsequently converted to RF energy at an output of the amplifier device. Velocity modulation works by alternately accelerating and decelerating a beam of electrons passing through an RF-driven input structure, such as a cavity or traveling-wave circuit. As the electrons drift downstream, their velocity differences cause them to group at the RF frequency. In contrast, density modulation works by RF gating the electron flow directly from the cathode surface, accelerating the resulting electron bunches, and extracting power using an output section. As a consequence, density-modulated devices are generally considerably shorter than their velocity-modulated counterparts. Additionally, because electron emission is controlled by the RF drive level, density-modulated devices retain a high degree of efficiency even when operated in the linear region.
To convert the modulated electron beam into microwave radiation, the electron bunches are passed through an appropriate output circuit that generates an RF current in response to the electron beam. At very high frequencies, conventional linear-beam output circuits are necessarily very small. This is problematic because the small physical size complicates fabrication and limits power-handling capability of the device.
Accordingly, it is desirable to provide an output circuit for a microwave tube amplifier that is physically large for a given frequency, thereby allowing ease of manufacture. It is further desirable to provide an output circuit that has generally high interaction impedance for good efficiency, and that has high average power capability.